This invention relates to compounds useful for complexing halogens and more particularly to compounds useful, either alone or in admixture, as an electrolyte additive for electrochemical cells and batteries of the type which utilize a halogen as its electrochemically active agent.
Voltaic cells which include an aqueous solution of metal halide, e.g., cadmium or zinc halide, as an electrolyte are known, but are frequently characterized by a relatively high self-discharge rate, low capacity, and high internal resistance. Because elemental halogen is soluble in the aqueous electrolyte, it is difficult to keep metallic zinc or cadmium and elemental halogen apart while simultaneously achieving a system in which a good percentage of the theoretical energy storage capacity can be realized.
Various attempts have been made to prevent elemental halogen from migrating to the zinc or cadmium electrode. For example, U.S. Pat. Nos. 2,566,114 and 3,373,058 to M. R. Bloch teach the use of quaternary ammonium halides in zinc or cadmium halogen cells. However, the salts suggested for use in such cells have cations which have tetrahedral symmetry (nitrogen cation symmetry number equal to twelve), exist as a crystalline material when complexed with halogen, and exhibit poor halogenation-dehalogenation kinetics. While such crystalline quaternary ammonium halides are capable of reducing the concentration of dissolved halogen during cell charge and discharge, because of their crystalline nature, they are unable to rapidly complex and release halogen and to remain concentrated in the vicinity of the current collector during cell cycling.
U.S. Pat. No. 3,816,177 to M. A. Walsh teaches the use of soluble quaternary ammonium halides and the like which are dissolved in the electrolyte together with a water soluble depolarizer. When elemental halogen is released into the electrolyte, it combines with the quaternary halide to form a quaternary polyhalide which in turn complexes with the depolarizer to form an insoluble, halogen rich, liquid complex. If an inert electrode made of a material which absorbs the insoluble complex is employed, an improved cell is provided because the liquid complex, being fluid, permits rapid diffusion of halogen during cell charge and discharge and because the complexed halogen molecules, being concentrated about the current collector, are available for electrochemical reaction to an improved degree. While this system represents a significant improvement over the use of crystalline quaternary polyhalides, it suffers from the requirement that the depolarizer, which cannot directly complex halogens, must be part of the liquid complex.
Further improvements in halogen electrochemical cells are disclosed in U.S. Pat. No. 4,038,459 to A. M. Ajami et al. entitled Halogen Complexing Alcohols and Nitriles and in U.S. Pat. No. 4,038,460 to F. M. Walsh et al. entitled Halogen Complexing Ethers. The water-soluble alcohols, ethers or nitriles disclosed in these patents form liquid polyhalides in the presence of elemental halogens or quaternary ammonium-halogen complexes which polyhalides are insoluble and halogen-rich. While cells and batteries containing these types of additives have certain advantages, they suffer from the temperature dependent reaction of alcohols, ethers or nitriles with the free halogens and hydrogen ions present in the aqueous zinc or cadmium halide electrolytes.
Another approach to the problem of improving the performance of halide cells is disclosed in U.S. Pat. No. 4,065,601 entitled Two Phase Electrolytes Used as Halogen Traps in Metal Halogen Secondary Cells. This application teaches the use of two phase electrolyte comprising an aqueous phase and a water immiscible organic phase. A halogen complexing organic salt such as an ammonium, pyridinum, sulfonium, or phosphonium salt is dissolved in the organic phase. This system depends on gravity separation of the phases of the electrolyte or on the ability of the halide electrode to absorb the organic phase.
The instant invention constitutes a further improvement in the halogen cell art and provides a large number of compounds, one or more of which may be added to the electrolyte of halogen cells of the type described. The compounds of the invention eliminate the requirement of adding a depolarizer or other additional additive to the electrolyte yet significantly increase the useful temperature and hydrogen concentration range at which the cells can successfully operate, provide increased halogen complexing ability, and thus enhance the shelf life and capacity of the cells and batteries in which they are used.